Electroless copper plating polydopamine nanoparticles

ABSTRACT

Aqueous dispersions of artificially synthesized, mussel-inspired polydopamine nanoparticles were inkjet printed on flexible polyethylene terephthalate (PET) substrates. Narrow line patterns (4 μm in width) of polydopamine resulted due to evaporatively driven transport (coffee ring effect). The printed patterns were metallized via a site-selective Cu electroless plating process at a controlled temperature (30° C.) for varied bath times. The lowest electrical resistivity value of the plated Cu lines was about 6 times greater than the bulk resistivity of Cu. This process presents an industrially viable way to fabricate Cu conductive fine patterns for flexible electronics at low temperature, and low cost.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a Division of U.S. patent application Ser.No. 14/965,468, filed Dec. 10, 2015, now U.S. Pat. No. 10,294,567,issued May 21, 2019, which is a Non-provisional of, and claims benefitof priority under 35 U.S.C. § 119 from, U.S. Provisional PatentApplication No. 62/090,668, filed Dec. 11, 2014, the entirety of whichare each expressly incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to the field of electroless plating, andmore particularly electroless plating on a substrate to form conductivepatterns.

BACKGROUND OF THE INVENTION

Emerging flexible electronic devices have exhibited significantpotential for a wide range of applications such as solar cells (1),batteries (2), sensors (3), antennas (4), and displays (5). For anyflexible electronic application, an essential characteristic iselectrically conductive patterning. Solution-based additivemanufacturing techniques such as drop-on-demand (DOD) inkjet printing(6), slot die coating (7), and gravure printing (8) are being widelyinvestigated to fabricate flexible conductive patterns. DOD inkjetprinting is an excellent candidate because it is amaterial-conservative, low-temperature process and is easilyincorporated into large scale roll-to-roll (R2R) manufacturinginfrastructures for flexible polymer substrates.

The commonly used ink materials in DOD inkjet printing processes can becategorized into two types: metal nanoparticle (NP) dispersions andmetal precursor solutions (9, 82). NP inks consist of metallic NPs and acarrier liquid solvent. The NPs have specifically designed surfaceproperties which allow them to be stably dispersed in an appropriatesolvent. Precursor ink is an inorganic metal salt or organic metalcomplex that is dissolved in a solvent. After inkjet printing, thesolvent undergoes evaporation and metal NP or precursor deposits on thesubstrate. A post-printing process is typically necessary to rendereither type of deposited structure electrically conductive. Polymercapping layers and surfactants are employed in the formulation of NPinks to prevent aggregation and particle precipitation; these agents aregenerally neither conductive nor volatile. The post-printing processremoves these agents and initiates sintering, thereby improving theelectrical conductivity. For precursor inks, the post-printing processchemically reduces the metal species from its ionic to elemental statewhich is electrically conductive. The standard post-printing processapplies heat that potentially puts substrate materials at risk ofthermal degradation/deformation, especially when low-cost polymericsubstrates are used (e.g., polyethylene terephthalate (PET), etc.). Afew non-thermal or local surface thermal techniques have been reportedincluding plasma (10, 11), laser (12), electrical (13), and photonic(14) methods. However, sophisticated equipment and their associatedhigh-cost processes are inevitable.

Drop-on-demand inkjet printing is a material-conservative depositionprocess compatible with the low temperature requirements of flexiblepolymer substrates (81). The implementation of printing within aroll-to-roll (R2R) infrastructure enables continuous, high-speed andlarge-scale manufacturing.

Electroless plating generally uses a solution of metal salt, reducingagent, a complexing agent, and additive(s) (such as bath stabilizer andplating rate adjusting agent)(20). Metal nucleates on the catalyticallyactive surface and continues to promote further metal reduction andgrowth. This is the defining characteristic of ELP's autocatalyticnature (20). A pre-patterned catalyst layer on the target substrate willpromote site-selective deposition. Hidber et al. (86) utilizedmicrocontact stamping to pattern palladium (Pd) colloids which yieldedcopper (Cu) patterns after ELP. Harkness et al. (87) usedphotolithography to pattern a Pd-bonded-organic seed layer (hydrogensilsesquioxane) and achieved site selective ELP of Cu and Ag. Theprohibitively large cost of photolithography has prompted research intothe use of inkjet printing to directly pattern the catalyst (88, 89, 90,91, 92, 93, 94). Most of the inkjet printing studies exploited Pd-basedink (88, 89, 90, 91, 92) while the investigation of other ink materialsis less focused (93, 94). Although Pd-based catalyst is versatile forelectroless plating of a wide range of metals, the high cost of Pdlimits its use (95).

Silver (Ag) is the most broadly investigated conductive ink material dueto its low bulk resistivity (1.6×10⁻⁸ Ωm) and resistance to oxidation;however, like other noble metals, it is expensive ($0.708/gram) (15).Copper (Cu) is preferred because it exhibits a bulk resistivity(1.7×10⁻⁸ Ωm) comparable to Ag but is significantly cheaper($0.007/gram) (15). However, the main challenge of using Cu-based rawmaterial for inkjet printing arises from the spontaneous formation of Cuoxides; when synthesized Cu NPs oxidize, both their resistivity andsintering temperature increase dramatically (9). Research efforts toovercome the Cu NP oxidation have taken two directions: utilizing anorganic oxygen barrier material as particle capping layers to retardoxidation kinetics (16) and synthesis of Cu-noble metal core-shell NPsto achieve long-term stability (17). Cu precursor inks are usuallystable against oxidation under room environment (18). Notwithstanding,the postprinting process for both Cu NP and Cu precursor must beimplemented in reductive, inert atmospheres or under vacuum to preventoxide formation, which inherently increases process complexity (19).Site-selective Cu electroless plating (ELP) is a method that can be usedto fabricate conductive patterns on flexible substrates. It is a lowtemperature process that does not cause substrate damage if a properplating bath is used. During ELP, formation of Cu oxides is dramaticallyinhibited. The general ELP uses a solution of metal salt, complexingagent, reducing agent and additive(s) (such as a bath stabilizer and apH adjusting agent) (20).

Site-selective ELP can be achieved by plating a substrate which has aprepatterned catalyst/seed layer. Studies have been conducted exploringinkjet printing for ELP seed patterning (21, 22, 23). The Pd-based inkis the most widely investigated material due to its well-establishedcatalytic activity for initiation of various metal deposition from awide range of ELP solutions (24).

Poly(dopamine) (PDA), a marine mussel inspired polymer, was recentlyfound capable of initiating metal ion reduction indicating its potentialas an ELP catalyst (26, 67, 96). PDA exhibits universal adhesion asdemonstrated for a wide range of both organic and inorganic materials(26). PDA can be synthesized as continuous coatings on any object byinducing dopamine polymerization in a pure water phase (26) or assuspended spherical NPs in water-alcohol mixtures at controlled pH (27).

Poly(dopamine) nanoparticles (PDA-NP) have been inkjet printed on bothglass and PET substrates followed by site-selective Ag ELP (28). Resultsexhibit a substrate-independent method to fabricate highly conductive Agpatterns.

The coffee ring effect is due to capillary flow induced by thedifferential evaporation rates across the drop: liquid evaporating fromthe contact line (“CL”) region is replenished by liquid from theinterior (29). When the CL is pinned and maximum evaporation occurs ator near the CL, the mass flow of solvent causes an accumulation ofsolute in the CL region. The resulting bulk flow toward the CL cantransport nearly all the dispersed material to the CL region. Thecooling due to evaporation induces a Marangoni flow inside a dropletopposing the evaporatively-driven flow. The Marangoni flow, if strong,favors particle deposition at the center region of the droplet. Thus,for particles to accumulate in the CL region, the liquid must have aweak Marangoni flow, or something must occur to disrupt the flow. Forexample, surfactants can be added to reduce the liquid's surface tensiongradient, disrupting the Marangoni flow. Water has an intrinsically weakMarangoni flow, a flow that is then reduced significantly by naturalsurfactants.

SUMMARY OF THE INVENTION

The present technology provides site-selective Cu plating oninkjet-printed PDA-NP lines on polyethylene terephthalate (PET)substrates.

It is an object to provide a method for forming a conductive trace on asubstrate wettable by a volatile liquid, comprising: providing asuspension of nanoparticles in the volatile liquid, the nanoparticlescomprising a catalyst for electroless plating; selectively depositingthe suspension of nanoparticles in the volatile liquid on the substratein a pattern, wherein at least a portion of the substrate remains dry;drying the volatile liquid, to form a pattern of nanoparticles on thesubstrate; and selectively electroless plating the nanoparticles, toform a conductive metal pattern corresponding to the selectivedeposition pattern of the suspension of nanoparticles.

It is also an object to provide a substrate having at least oneconductive trace, formed by a process comprising: providing a substratehaving a surface wettable by a volatile liquid; selectively depositing asuspension of nanoparticles, the nanoparticles comprising a catalyst forelectroless plating, on the surface in a pattern, wherein at least aportion of the substrate remains free of the suspension; drying thevolatile liquid, to form a pattern of nanoparticles on the substrate;and selectively electroless plating the nanoparticles, to form aconductive metal pattern corresponding to the selective depositionpattern of the suspension of nanoparticles on the surface.

It is a still further object to provide a substrate having electricallyinterconnecting conductive traces, comprising: a surface wettable by anaqueous solution; a plurality of catalytic nanoparticles deposited onthe wettable surface in a pattern of parallel lines separated by aregion of sparse catalytic nanoparticle deposition; and an electrolessplating of a conductive metal selectively forming conductive traces overthe parallel lines of catalytic nanoparticles, and being electricallyinsulating between the respective parallel lines.

It is another object to provide a device, comprising: a substrate havinga surface, at least a portion of which are hydrophilic; a pattern ofdeposited catalytic nanoparticles on the surface, comprising a patternof parallel lines spaced by a region of sparse catalytic nanoparticledeposition; and electrolessly plated metal selectively formed proximateto the catalytic nanoparticles, forming conductive traces over theparallel lines of catalytic nanoparticles, and being electricallyinsulating between the respective parallel lines.

The catalytic nanoparticles may comprise polydopamine, and theelectrolessly plated metal may comprise copper.

The substrate may comprise a polyimide sheet, or glass, for example. Thedevice may be used to replace indium-tin-oxide transparent conductors,for example in flat panel displays, solar cells, touchscreens, and thelike. These conductors may also be used to form electrodes for thin filmtransistors and other active devices, such as light emitting diodes andorganic light emitting diodes. The substrate may have various sets ofparallel lines formed on it, in various patterns, such as linear arrays,rectangular arrays, spirals, or other geometric pattern or arbitraryconfigurations. The parallel lines may advantageously be used aselectrodes between ends of carbon nanotubes and other “1-dimensional”structures. In some cases, the fibers may be present in the volatileliquid, and self-align during evaporation across the gap between theedges. In other systems, the fibers may be provided after theelectroless copper plating (or other metallic plating) is conducted.

The conductive lines may be used as electrodes in an electrowetting lenssystem, in which the meniscus formed between two liquids is changed inshape based on electrical potential, and thus providing electronicallyvarying focal length. Further, the technology may also be used in othertypes of electro-optics, and human eyewear, to provide effectivelytransparent conductive functionality. For example, anelectrically-controlled polarizer may be provided, for 3D televisionlight shutter glasses and optical sunglasses (for example, which can benormally polarized, but “turned off” to view a cell phone or automotivedisplay).

The technology also permits printing of the volatile solution suspensionof catalytic nanoparticles. For example, an aqueous solution ofpolydopamine particles may be deposited from a print head (e.g., inkjet, bubble jet, extrusion) as part of an additive manufacturingprocess. The volatile solvent is permitted to evaporate, forming thedouble line structure by virtue of the coffee ring effect. Note that itis also possible to form a filled pattern by avoiding fulfilling theconditions that lead to a sparse gap between the edges. An electrolessplating solution may then be sprayed over the catalytic nanoparticlepattern, for form a conductive structure. The electroless solution maythen be washed away, and further manufacturing steps conducted. Theresulting device may, using appropriate additive manufacturingmaterials, be optically clear, and therefore the conductors may beembedded in a lens or window. The manufacturing process may deposit, forexample, liquid crystal materials, organic light emitting materials, orthe like to form functional devices such as polarizers, light shutters,displays, etc.

A printed device may be, for example, a “lab on a chip” design, in whichthe parallel lines may act as one or two electrodes of a sensor orconnecting to a sensor material or device.

The line pattern may also be used to produce an antenna or inductor, orcomplex devices such as electronic filters and transformers.

The line patterns may also be used as embedded sensors, such ascapacitive, stress and strain, piezoelectric, pyroelectric, etc. Thus,for example, a polyvinylidene fluoride (PVDF) or vinylidene fluoride(VDF), trifluoroethylene (TrFE) substrate may be coated with sensinglines, to provide a room temperature thermal movement imaging sensor, atlow cost.

The technology may also be used for produce “smart windows”, for examplewith electrochromic darkening.

The nanoparticles may comprise polydopamine or other catalytic material.

The suspension of nanoparticles may be selectively deposited by inkjetprinting, lithography, or a pad printing process.

The suspension of nanoparticles may be aqueous, and selectivelydeposited by selectively forming hydrophobic and hydrophilic regions onthe substrate, and the hydrophilic regions wet with the suspension. Thevolatile liquid may be aqueous, and the drying may comprise evaporatingthe suspension in air.

The electroless plating may comprise plating the nanoparticles withsilver, copper, gold, or nickel, for example.

The selectively depositing the suspension of nanoparticles may compriseinkjet printing a line, evaporating the volatile liquid to redistributethe nanoparticles exploiting the spontaneous coffee ring effect toinduce a higher concentration of nanoparticles at the edges of theinkjet-printed line and a sparse distribution of nanoparticles betweenthe edges.

The electroless plating, e.g., over the coffee ring-effect separatedpairs of parallel lines of catalytic nanoparticles may form a pair ofseparated conductive traces at the edges of the inkjet-printed line.

The electroless plating may comprise electroless copper plating,resulting in a continuous reduced copper trace on the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B shows typical scanning electron microscope (SEM) imagesof printed lines of polydopamine nanoparticles (FIG. 1A) and linessubject to 15 min copper electroless plating (FIG. 1B).

FIGS. 2A-2C show structural and electrical performance characterizationof the deposit, in which FIG. 2A shows Cu thickness (τ) evolution; FIG.2B shows individual twin line resistance; and FIG. 2C shows individualtwin line resistivity change as a function of Cu electroless platingtime (t).

FIG. 3 shows X-ray diffraction results of the printed polydopaminenanoparticle arrays before and after 15 min Cu electroless plating. Thedotted lines indicate 2θ values of face-centered-cubic copper accordingto the Joint Committee on Powder Diffraction Standards (JCPDS) file04-0836.

FIGS. 4A-4D show robustness characterization of the inkjet printedpolydopmaine nanoparticle (PDA-NP) lines followed by 15 min Cuelectroless plating, in which FIG. 4A shows twin line resistance changeafter cyclic bending; FIG. 4B shows results of a tape peel adhesiontest; FIG. 4C shows the influence of air exposure time on plated lineresistance (R) relative to its as-plated resistance (RO) (the inset isthe first 4 h air exposure result as indicated by the arrow); and FIG.4D shows a SEM images show the formation of cracks after 10,000 bendingcycles with 2.5 mm bending radius.

FIG. 5 shows a typical dynamic light scattering (DLS) particle sizeanalysis result according to the embodiment described in Example 2.

FIG. 6 shows schematic of the mask used for electrode sputteringdeposition.

FIGS. 7A-7C shows stages of a process according to the presenttechnology; FIG. 7A shows a polydopamine nanoparticle suspension isinkjet printed on a substrate, and forms a domed pattern correspondingto the deposition droplet locations; FIG. 7B shows the pattern after theliquid dries, wherein the coffee ring effect causes the nanoparticles toselectively deposit the nanoparticles at the edges of the originalliquid range; and FIG. 7C shows the result after the polydopaminenanoparticles are electrolessly plated with copper.

FIG. 8 shows dynamic light scattering characterization of the sizedistribution of synthesized polydopamine nanoparticles according to theembodiment described in Example 3.

FIGS. 9A-9D show a scheme for forming conductive traces on a substrate,in which FIG. 9A shows poly(dopamine) nanoparticles (PDA-NP) that wereprepared as aqueous ink and inkjet printed on glass substrate; FIG. 9Bshows a pattern of PDA-NP formed after evaporation; FIGS. 9C and 9D showsilver electroless plating for short and long time, respectively.

FIGS. 10A-10C show typical microscopic images of inkjet printedpolydopamine nanoparticle (PDA-NP) lines as a function of silverelectroless plating (Ag ELP) time, showing top-view optical images ofas-deposited PDA-NP lines, after 24 min (FIG. 10A) and 120 min (FIGS.10B, 10C) Ag ELP, respectively; the exploded views show top andcross-section SEM images at the indicated regions. The SEM scale barsare 500 nm. The SEM labels T and X refer to top and cross-sectionalview, respectively.

FIGS. 11A-11C show deposited structural characterizations of: FIG. 11A,individual-line resistance (R) as a function of silver electrolessplating time, FIG. 11B, thickness (τ) evolution at different locations;FIG. 11C, electrical resistivity change at the edges and interlines. Theinserts of (c) are the hypothesized parallel resistor models. Thesubscript e and i refer to edge and interline, respectively.

FIG. 12 shows XRD patterns of the inkjet printed PDA-NP arrays as afunction of Ag ELP time: The dotted lines indicate 2θ values offace-centered-cubic Ag according to the Joint Committee on PowderDiffraction Standards (JCPDS) file 04-0783

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1

PDA-NP was synthesized according to a published protocol (27) with minormodification. The ink was formulated by mixing PDA-NP with distilleddeionized (DI) water at a concentration of 0.25 wt % followed by 10 minultrasonication (3510, Branson). The particle size distribution and zetapotential were characterized by dynamic light scattering (DLS)(ZetasizerNano, Malvern). The PDA-NP average diameter was about 340±55nm. The zeta potential of PDA-NP was measured as −31.0±0.5 mV,indicative of good stability of the ink dispersion. Particleprecipitation was not observed over a 30-day period.

The viscosity and surface tension were measured as 1.0±0.1 mPa·s by arotary viscometer (TA1000, TA Instruments) and 72.1±0.1 mN/m by a bubbletensiometer (BP100, Kruss), respectively. These values were close tothose of pure water due to the very small PDA-NP loading.

FIGS. 1A and 1B show typical scanning electron microscope (SEM) imagesof printed lines of polydopamine nanoparticles (FIG. 1A) and linessubject to 15 min copper electroless plating (FIG. 1B). The linesindicate the approximate regions that are shown at higher magnificationfrom both top and cross-sectional perspectives. The arrows in thecross-sectional SEM image of the copper plated lines indicatepenetration of copper into the interstitial space of the polydopaminenanoparticle layer.

A single-nozzle inkjet printer was used to print the PDA-NP suspensiononto plasma-treated PET substrates (Melinex ST506, Dupont) to formarrays of five lines. After printing and solvent evaporation, PDA-NPsassembled into a pair of continuous lines with a minimal amount ofnon-continuous deposition within the region between the line pair (FIG.1A). This phenomenon is caused by a convective flow driven bynon-uniform evaporation from the liquid/air interface, and wasextensively studied for drops and referred to as the “coffee ringeffect” (29). In our previous study, we defined this pair of lines astwin lines due to their similarity (28, 30). The width of a typicalas-printed PDA-NP twin line was measured as 3.9±0.6 μm in this study.

A potential mechanism of PDA-induced ELP has been suggested to involvethe electrostatic interaction of metallic ions with surface catecholgroups of PDA followed by catechol oxidation to quinone and reduction ofmetallic ions to elemental metal (26). The newly deposited elementalmetal further catalyzes the redox reaction for continuous deposition. Inthis study, Cu ELP was performed by immersing the printed PDA patternsin a chemical bath at 30° C. for different durations.

The morphology of the printed twin line before and after the platingprocess was characterized by scanning electron microscopy (SEM) (Supra55VP, Zeiss) from both top and cross-sectional perspectives. For theas-deposited lines, each twin line typically consists of asingle-particle thick PDA-NP structure. After 15 min of Cu ELP, a layerof Cu was plated on top of the PDA-NPs with some penetration into theinterstitial space of the PDA-NP layer (FIG. 1B). The plated Cu layerexhibited continuous structure with large grain size. It is also worthnoting that no line width broadening was observed after Cu ELP.

FIGS. 2A-2C show structural and electrical performance characterizationof deposit. FIG. 2A shows Cu thickness (τ) evolution. FIG. 2B showsindividual twin line resistance. FIG. 2C shows individual twin lineresistivity change as a function of Cu electroless plating time (t).

Cu thickness was measured by the analysis of the cross-sectional SEMimages. The thickness value is exhibited in FIG. 2A based on the averageof at least 8 different cross-sectional SEM pictures at each ELP time.The metal thickness increased linearly with increasing ELP time. Theplating rate of the Cu ELP process was measured to be about 42 nm/min. Amaximum thickness of 682±23 nm was achieved at the 15 min plating time.

The electrical resistance of each array of plated lines was assessed bya method we previously reported (28, 30). The average resistance of eachtwin line after ELP processes is shown in FIG. 2B. The minimum timerequired to achieve a conductive structure was approximately 5 min witha large resistance value of about 7722Ω. The resistance decreasedsignificantly to about 724Ω for an ELP time of 10 min. The resistancethen decreased slowly with increased plating time to a final resistanceof 165Ω for an ELP time of 15 min. The resistivity of each twin line wascalculated according to ρ=RwτL⁻¹ where ρ is the resistivity, R the lineresistance, w the line width, τ the thickness, and L the length,respectively. The calculated resistivity values are shown in FIG. 2C.The resistivity values decrease with increasing ELP time and the trendis similar to the resistance change shown in FIG. 2B. The lowestresistivity value achieved after 15 min ELP was 1.1×10⁻⁷ Ωm, a valueapproximately 6 times that of bulk Cu.

FIG. 3 shows X-ray diffraction results of the printed polydopaminenanoparticle arrays before and after 15 min Cu electroless plating. Thedotted lines indicate 2θ values of face-centered-cubic copper accordingto the Joint Committee on Powder Diffraction Standards (JCPDS) file04-0836.

Materials composition was confirmed by X-ray diffraction (XRD) (XDS2000,Scintag) as shown in FIG. 3. The broad diffraction peaks occurring at 2θangles of approximately 47 and 54° are attributable to the PETsubstrates (31). The existence of printed PDA-NP lines does notintroduce any additional diffraction patterns. For the patterns after 15min Cu ELP, two characteristic diffraction peaks were exhibitedassociated with face-centered-cubic Cu crystalline at 43 and 51° (for Cu(111) and (200) planes, respectively). The 2θ locations of thediffraction peaks are consistent with the Joint Committee on PowderDiffraction Standards (JCPDS) file 04-0836. The XRD characterizationswere all conducted within 30 min of the completion of the Cu ELPprocess. The absence of the diffraction peaks of cuprous or cupric oxideis perhaps due to the small amount of native oxide that is undetectable.Additional oxide characterization was beyond the primary focus of thisstudy.

FIGS. 4A-4D provide robustness characterization of the inkjet printedpolydopmaine nanoparticle (PDA-NP) lines followed by 15 min Cuelectroless plating. FIG. 4A shows Twin line resistance change aftercyclic bending. FIG. 4B shows results of a tape peel adhesion test. FIG.4C shows the influence of air exposure time on plated line resistance(R) relative to its as-plated resistance (RO) (the inset is the first 4h air exposure result as indicated by the arrow). FIG. 4D shows SEMimages revealing the formation of cracks after 10,000 bending cycleswith 2.5 mm bending radius per FIG. 4B.

The bending robustness of Cu patterns was assessed using a cyclicbending test. During each cycle, the substrate was cyclically bentbetween concave and convex geometries with known radius of curvature(2.5 mm or 5 mm). The number of cycles ranged from 1000 to 10000, whilethe bending frequency was maintained at 100 cycles/min. The resistanceof each line versus number of bending cycles is shown in FIG. 4A. Allthe test samples were plated for 15 min. Only a slight increase ofresistance with cycles was observed when the 5 mm bending radius wasapplied (FIG. 4A). The resistance increases from the as-plated value 165to 170Ω (about 5% increase). For the 2.5 mm bending radius, theresistance value increased to 187Ω, a 13% increase compared to the valueprior to bending. The resistance increase due to bending is likely dueto the formation of cracks as shown in FIG. 4A (SEM images).

The adhesion of the plated metal lines was assessed qualitatively by atape peel test. Briefly, a tape (Scotch600, 3M) was carefully adhered tothe patterns and subsequently removed by peeling it off parallel to thesubstrate. Then the remaining patterns were observed by an opticalmicroscope (AxioObserver Al, Zeiss). The plated lines (15 min Cu ELP)demonstrate good adhesion on PET substrates (FIG. 4B). Subsequent toadhesive tape attachment and removal, no evidence of delamination orstructural change was observed. Furthermore, there was no apparentdegradation of adhesion after 10000 bending cycles.

FIG. 4C shows resistance as a function of air exposure time in a roomenvironment. The Cu lines were formed by ELP for 15 min. Cu lineresistance increases gradually to a value 1.43 times its as-plated valueafter 720 h, likely due to the formation of Cu oxides when elemental Cuis exposed to air. The resistance within the first 4 h of air exposureshows no increase relative to its as-plated resistance (FIG. 4C, inset),confirming that the Cu line resistance increase shown in the cyclicbending test (FIG. 4A) was not influenced by air exposure, as thebending test and the resulting resistance measurement were completedwithin 2.5 h.

FIG. 7 shows stages of a process according to the present technology. Onthe left, a polydopamine nanoparticle suspension is inkjet printed on asubstrate, and forms a domed pattern corresponding to the depositiondroplet locations. After the liquid dries, the coffee ring effect causesthe nanoparticles to selectively deposit the nanoparticles at the edgesof the original liquid range. On the right, the polydopaminenanoparticles are electrolessly plated with copper.

Therefore, fine lines of Cu with low electrical resistivity werefabricated by sequential inkjet printing of an aqueous dispersion ofmussel-inspired poly(dopamine) nanoparticles and site-selectiveelectroless plating. The process forms Cu lines with thickness linearlydependent on plating time, and with an electrical resistivity 6 timesthat of bulk Cu. The resistance of the Cu lines does not changesignificantly with cyclic bending characterized by a 5 mm bendingradius. While resistance of Cu lines is subject to increase overlong-time storage, probably because of oxidation, the conductivityobserved is reasonably stable for at least one month, giving reasonabletime to manufacture a complete system and protect the lines from furtherdegradation. This process demonstrates a simple, low temperature andlow-cost method of fabricating fine conductive Cu patterns for flexibleelectronics applications.

Example 2

Materials

Dopamine hydrochloride (98.5%, Sigma Aldrich), Tris (99.9%, SigmaAldrich), 2-propanol (99.5%, Fisher Scientific), silver nitrate (99%,Sigma Aldrich), ethylenediamine (98% m Fisher Scientific), potassiumsodium tartrate tetrahydrate (99%, Fisher Scientific),3,5-diiodotyrosine (99%, Sigma Aldrich), copper chloride dehydrate (99%,Fisher Scientific), ethylenediamine (99%, Fisher Scientific), ascorbicacid (99%, Fisher Scientific), potassium bromide (99%, FisherScientific), cobalt chloride hexahydrate (98%, Fisher Scientific),2,2′-Bipyridal (98%, Fisher Scientific), hydrochloric acid aqueoussolution (1M, Sigma Aldrich) and sodium hydroxide (99%, FisherScientific) were used as received. Water was distilled subsequent todeionization (DI) prior to use.

Polydopamine Nanoparticle (PDA-NP) Synthesis

PDA-NP powder was synthesized according to a published protocol (27)with minor modification. Tris was dissolved in distilled DI water at 10mM to form 250 ml solution to which 2-propanol was subsequently added.Magnetic stirring was applied at 300 rpm for 30 min to ensure a uniformmixture. 125 mg dopamine chloride was added to this mixture, and keepsbeing stirred at the same condition for 72 hrs to form the PDA-NPsuspension. Purification was necessary to remove excessive reactant andTris. This was achieved by eight cycles of 10,000 rpm centrifugation(5810R, Eppendorf) for 10 min at 23° C., decantation of upper liquid andre-dispersion of settled solid in fresh distilled DI water by vigorousshaking. The synthesized PDA-NP was dried under a mild nitrogen streamovernight.

PDA-NP Diameter Characterization

The PDA-NP diameter was characterized by dynamic light scattering (DLS).The result is shown in FIG. 5.

Substrate Preparation

PET (Melinex ST 506, Dupont) substrates thoroughly rinsed by copiouswater and 2-proponal alternatively. The substrate surfaces wereprocessed by oxygen plasma to improve hydrophilicity using a plasmatreatment system (PE200-RIE, PlasmaEtch). Before plasma generation, thechamber was vacuumed to less than 5 mtorr. Then ultra-high pure oxygenwas introduced to the chamber at flow rate of 20 sccm while maintainingthe final pressure at 100 mtorr. After the flow rate and pressure werestabilized for 10 min, oxygen plasma was generated at 100 W radiofrequency (13.56 Hz) power for 1 min. The auto-tuning network was usedto ensure the maximum reflected power less than 1 W along the entireplasma duration. The substrates were used immediately after plasmaprocessing.

Drop-On-Demand (DOD) Inkjet Printing Process

The DOD inkjet printing system used in this study is a custom-designedapparatus with a single piezoelectric printing nozzle of 80 μm orificediameter (Microfab). The nozzle is actuated by a waveform generator(JetDrive III, Microfab) and positioned by a computer-controlled x-ystage (MX80L, Parker). Drop formation was observed along an optical axisperpendicular to the nozzle with a camera (Sensicam QE, Cooke Corp.) tomaintain drop diameter (55±2 μm), speed (1±0.2 m/s), and ensure absenceof satellites. Drop frequency was controlled at 100 Hz by adjusting thefrequency of the waveform generator. The centre-to-centre distancebetween two adjacent printed droplets was fixed at 50±2 μm andcontrolled by the speed of the inkjet nozzle over the substrate (5mm/s). The printed line length was controlled at 6 mm. Five lines wereprinted to form an array. And four arrays were on each substrate.

Electroless Plating (ELP) Process

Distilled DI water used for Cu ELP process was deoxygenated by bubblingnitrogen gas over 24 hrs. The Cu ELP bath comprises aqueous solutions of50 mM copper chloride dehydrate, 600 mM ethylenediamine, 150 mM cobaltchloride hexahydrate, 0.04 mM potassium bromide and 0.04 mM2,2′-Bipyridal. The PH value of the as-prepared bath was adjusted to9.4±0.1 by adding appropriate amounts of sodium hydroxide andhydrochloric acid solutions. During plating, the bath PH was maintainedwithin the range 9.4 and 9.5 by adding hydrochloric acid at 0.2 ml/minwith a syringe pump. The ELP solution temperature was controlled at 30°C. using a water bath. The solutions were mechanical stirred at 100 rpmduring the entire plating period.

Electrical Resistance Characterization

The resistance of each array of printed and plated silver lines wasfacilitated by depositing two silver electrodes at the ends of the arraysuch that the printed lines spanned a fixed distance (4 mm) between theelectrodes. One electrode pair was deposited over each line array byshadow sputtering silver through machined aluminium mask (FIG. 6).Kapton tape was used to provide ˜40 μm standoff distance between themask and the printed structure. Alignment of the mask to an array wasrealized with the help of an optical microscope.

The sputter deposition process was carried out with a magnetronsputtering system (CRC-600, Torr International) in DC mode. Thedeposition was controlled at 50 W with a flow rate of 100 sccm Argonresulting in 1.5×10⁻² torr working chamber pressure. Electrodes of ˜200nm thickness were deposited after 3 min sputtering.

The typical surface temperature of a deposited silver electrode wasmeasured as 29° C. by a thermocouple immediately after the chamber wasvented with nitrogen gas to atmospheric pressure (˜4 min ventingprocess). It is unlikely that the actual surface temperature duringsputter deposition was significantly higher than the measured value soas to have a measurable effect on the structure and electricalperformance properties of the printed and plated silver lines.

After electrodes deposition, a four-point-probe method was used tomeasure the resistance value. Each twin line resistance value can becalculated by sequentially cut a twin and recording the updatedresistance value using a parallel resistor model.

Example 3

Polydopamine nanoparticles (PDA-NP) were synthesized according to Yan etal. (27). PDA-NPs were generated by initiating dopamineself-polymerization in a water-isopropanol mixture which was buffered byTris. The average size of PDA-NP was measured with a dynamic lightscattering (DLS) system (zetasizer-nano, Malvern) to be 338±55 nm. Theas-synthesized PDA-NP powder was dispersed in distilled deionized (DI)water at 1% wt by 10 min agitation by ultra-sonication. The viscosityand surface tension of the PDA-NP ink were measured as 1.0±0.1 mPas and72.3±0.1 mN/m using a cone-plate rheometer (AR1000, Texas Instrument)and bubble pressure tensiometer (BP100, Kruss), respectively

pH buffer agent Tris was dissolved in distilled deionized (DI) water at10 mM to form 250 mL solution to which 80 mL of 2-propanol wassubsequently added. 125 mg dopamine chloride was added to this mixture,and agitated by magnetic stirring at 300 rpm for 72 hrs to form thePDA-NP suspension. Purification of the suspension was necessary toremove excess reactants and Tris buffer. This was accomplished by threecycles of centrifugation (3500 rpm for 1 hr at 23° C.) and re-dispersionof settled particulate matter in distilled DI water via vigorousshaking. The synthesized PDA-NP was dried under a stream of nitrogen.The ink was prepared by mixing the PDA-NP dry powder with distilled DIwater followed by 10 min ultrasonication. All chemicals were purchasedfrom Sigma Aldrich (USA) and used as received.

The size distribution of PDA-NP was characterized by dynamic lightscattering (Zetasizer-nano, Malvern). A representative size distributionof a 1% wt sample of ink is shown in FIG. 8.

Glass microscope slides (2947, Corning) were used as substrates and wereprepared as follows. All substrates were immersed in a Micro-90(Cole-Palmer) aqueous solution (˜1% v) and underwent ultrasonicagitation for 12 min. Copious rinsing with distilled DI was followed bydrying under a nitrogen stream. The substrate surface was then processedusing argon gas plasma treatment (PE200-RIE, PlasmaEtch) to increasesurface hydrophilicity. The plasma parameters were controlled at 100mtorr pressure and 100 sccm flow rate argon with 100 W radio frequency(13.56 Hz) power for 3 min. The substrates were used immediately afterplasma treatment.

The inkjet printing apparatus (30) consists of a single piezoelectricinkjet nozzle (80 μm diameter, Microfab) actuated by a waveformgenerator (JetDrive III, Microfab) and positioned via acomputer-controlled x-y stage (MX80L, Parker). Drop formation wasobserved along an optical axis perpendicular to the nozzle with a videomicroscope (Sensicam QE, Cooke Corp.) to maintain drop diameter (55±2μm), speed (1±0.2 m/s), and ensure absence of satellites. Drop frequencywas controlled at 100 Hz by adjusting the frequency of the waveformgenerator. The center-to-center spacing between two adjacent printeddrops was fixed at 50±2 μm and controlled by the traverse speed of theinkjet nozzle over the substrate (5 mm/s). To visualize materialsdeposition process along with evaporation, a top-view camera (Kodak ES1.0) and a bottom-view microscope (AxioObserver Al, Zeiss) wereequipped.

The electroless plating process followed a published report (11). Anaqueous bath consisted of 3.0 mM silver nitrate, 18 mM ethylenediamine,35 mM potassium sodium tartarate and 0.04 mM 3,5-diiodotyrosine. Thebath temperature was maintained at 30° C. Agitation was applied at 100rpm with mechanical stirring during the entire plating process.

FIGS. 9A-9D show a scheme for producing silver conductive traces on asubstrate. FIG. 9A shows poly(dopamine) nanoparticles (PDA-NP) preparedas aqueous ink and inkjet printed on a glass substrate. FIG. 9B showspattern of PDA-NP was formed after evaporation. FIGS. 9C and 9DD show Agelectroless plating for short and long time, respectively.

A single-nozzle inkjet printer was used to print the PDA-NP suspensiononto plasma-processed glass substrates (2947, Corning) to form arrays offive lines, as shown in FIGS. 9A-9B. Solvent evaporation during printingdeposits nanoparticles at the contact line of the printed fluidstructure which results in a pair of twin parallel lines with minimalparticle deposition in the interline region. Each twin line measured24±1 μm in width with 400±12 μm between each pair. This twin-linedeposition process is a form of the well-known evaporatively-driven“coffee ring” phenomenon (29). In a previous report, we demonstrated theutilization of this phenomenon to print highly conductive twin lines andrectilinear grids of silver nanoparticles (30).

Ag deposition onto the printed pattern can be achieved by simpleimmersion into a silver salt solution such as silver nitrate (AgNO₃)(26). However, an ELP process that includes an additional reducing agentis necessary to ensure the rapid formation of a continuous andsufficiently thick metal layer. A potential mechanism of PDA-induced ELPhas been suggested to involve the electrostatic interaction of metallicions with surface catechol groups of PDA followed by catechol oxidationto quinone and reduction of metallic ions to elemental metal. The newlydeposited elemental metal further catalyzes the redox reaction forcontinuous deposition. In this study, Ag ELP was performed by immersingthe printed PDA patterns in a chemical bath at 30° C. for differentdurations.

The morphology of the printed lines before and after the plating processwas characterized by optical microscopy (Axio Imager, Zeiss) andscanning electron microscopy (SEM) (Supra 55VP, Zeiss). Top andcross-sectional views show that for the as-deposited lines, each set oftwin lines consists of closely-packed PDA-NP structure that is 1-2particles thick (FIG. 10A). In contrast, the interline region of eachtwin line pair consists of few PDA-NP islands. After 24 min Ag ELP, aporous and small-grain Ag layer forms on top of each twin line. In theinterline region, Ag deposition occurred predominantly over the PDA-NPislands with minimal nucleation of Ag directly on the substrate (FIG.10B, top-view SEM at the interline region). The Ag continuity and grainsize increase with extended ELP time (FIG. 10C) at both twin andinterline regions. However, smaller grain size and less continuousstructures were always observed for the Ag grown in the PDA-NP-freeregion. SEM cross-sectional images show that, in general, ELP yields athin layer of Ag over the top surface of the PDA-NP deposit; somepenetration into the space between PDA-NPs can also be observed (FIGS.10B and 10C), cross-sectional view SEM at the twin region).

Cross-sectional SEM images were also analyzed to measure the Agthickness on each twin line and interline region as a function of metalplating time. Plating times shorter than 1 hr did not yield accuratethickness measurements of the interline region due to its discontinuity.In general, the Ag growth rate at the twin lines is linear and larger incomparison to that of the interline region (FIG. 11A).

The electrical resistance of Ag-plated lines was measured using a linescratching technique (74, 75). Briefly, a pair of Ag electrodes wassputtered through a mask over each line array so that the lines spannedthe distance between electrodes. Four-point-probe measurements werecarried out to determine the total resistance of the five lines. Eachline was carefully cut by a needle attached to a micro-positioner andthe new resistance value recorded. Under the assumption that each linearray is a parallel resistor network, this method allowed measuring theaverage resistance of each cut line. As FIG. 11B shows, single lineresistance decreases rapidly within 24 min (62.20Ω) of ELP, followed byslower decay on a longer time scale to reach the lowest value of 11.70Ωat 120 min. It was possible to determine whether the interline region ofeach twin line pair was conductive by cutting all the twin lines. It wasfound that for plating times longer than 58 min, an electricallyconductive path forms in the interline region.

For electroless plating time t_(plating)<58 min, only the pair of twinlines is conductive. The measured resistance (R_(m)) is expressed as:

$\begin{matrix}{\frac{1}{R_{m}} = {\frac{2}{R_{t}} = \frac{2}{\rho_{t}{L/\left( {W_{t}\tau_{t}} \right)}}}} & (1) \\{\rho_{t} = {{\frac{2R_{m}W_{t}\tau_{t}}{L}\mspace{14mu}{for}\mspace{14mu} t_{plating}} < {58\mspace{14mu}\min}}} & (2)\end{matrix}$

For t_(plating)≥58 min, both twin lines and the interline region areconductive so that the measured resistance is:

$\begin{matrix}{\frac{1}{R_{m}} = {{\frac{2}{R_{t}} + \frac{1}{R_{i}}} = {\frac{2}{\rho_{t}{L/\left( {W_{t}\tau_{t}} \right)}} + \frac{1}{\rho_{i}{L/\left( {W_{i}\tau_{i}} \right)}}}}} & (3)\end{matrix}$

where ρ_(i), W_(i) and τ_(i) are the interline region's resistivity,width and thickness, respectively.

The procedure used to measure ρ_(i) independent of ρ_(t) is demonstratedas follows. First, the combined resistance R_(m) was measured, then thetwin lines were mechanically sequentially scratched off and the newresistance R*_(m) recorded. With the measured width of interline region(W*_(i)) and its resistance (R*_(m)), the resistivity of the interlineregion is:

$\begin{matrix}{\rho_{i} = \frac{R_{m}^{*}W_{i}^{*}\tau_{i}}{L}} & (4)\end{matrix}$

Substituting Eq. 4 into Eq. 3, ρ_(t) can be written as:

$\begin{matrix}{\rho_{t} = {{\frac{2R_{m}W_{t}\tau_{t}}{{\rho_{i}L} - {W_{i}R_{m}\tau_{i}}}\mspace{14mu}{for}\mspace{14mu} t_{plating}} \geq {58\mspace{14mu}\min}}} & (5)\end{matrix}$

The term W may be replaced by W−2W_(t) where W is the width of theentire line.

The electrical resistivity of plated lines of silver was estimated basedon two observed structural regimes: i) for plating times shorter than 58min, a twin-line structure consisting of two parallel conductors with noelectrical contribution from the interline region and ii) a threeparallel-conductor model for t≥58 min (FIG. 11C inset). For thetwo-conductor structure, the resistivity (ρ_(t)) can be expressed asρ_(t)=2Rτ_(t)W_(t)/L (where R the measured resistance of an individualline (a pair of twin lines), τ_(t) the Ag thickness at the edge, W_(t)the width of twin and L the length between two electrodes). For thethree-conductor case, the electrical resistance of the interline regionwas measured by complete removal of the twin lines. The resistivity ofeach twin line can then be expressed asρ_(t)=2Rτ _(t) W _(t) /Lρ _(t)=2Rτ _(t) W _(t)ρ_(i/)/ρ_(i) L−(W−2W_(t))Rτ _(t)),

where ρ_(i) is the resistivity of the interline region; R the measuredresistance of an individual line (a pair of twin lines plus theinterline region); W the entire line width and τ_(i) the Ag thickness atthe interline area). FIG. 11C plots ρ_(t) and ρ_(i) values as a functionof ELP time. Within the first 24 minutes of plating, ρ_(t) reaches avalue which is 10× larger than the resistivity of bulk Ag. The firstmeasurable ρ_(i) occurs at 58 min indicating the onset of structuralcontinuity of the interline region. The large resistivity (3.12E-4 Ωm)at that moment can be explained by film's highly porous nature as alsoconfirmed by SEM images. The values of ρ_(i) decreased more than twoorders within only two minutes from 58 to 60 min. After longer platingtime, ρ_(i) decreases slowly. The lowest values for ρ_(i) and ρ_(t) are4.84×10⁻⁷ and 7.80×10⁻⁸ Ωm which are 30× and 5× comparing with the bulkAg resistivity value, respectively.

Crystallographic analysis of the deposited Ag was performed by X-rayDiffraction (XRD) (Equinox 2000, Inel) on each array. Diffractionpatterns after different plating times are displayed for in FIG. 12. Alarge diffraction peak around 25° indicates the contribution of theamorphous glass substrate. As-deposited PDA patterns did not display anycharacteristic diffraction peaks. The appearance of a strong peak at38.1° after 24 min plating indicates Ag (111) crystal planes. After 60min, the peak intensity at 38.1° increases and four additional peaksappear at about 44.1°, 64.4°, 77.3° and 81.4°, corresponding to Agcrystal planes of (200), (220), (311) and (222), respectively. Theintensity of all 5 peaks continues to grow with longer plating. Theemergence of characteristic Ag peaks and their increase in intensity isconsistent with Ag growth observed from SEM. The diffraction peaks'narrowing effect is also observed comparing the patterns collected at120 min with 60 min ELP time. It can be explained by Ag crystallineimprovement along with increasing ELP time which is again, implied bythe Ag continuity increase from SEM images. Here, we did not apply theScherrer's equation (76) to estimate the crystallite size due to thenature of discontinuous pattern, which influences the estimationaccuracy.

In conclusion, Ag lines of high electrical conductivity were fabricatedby sequential inkjet printing of mussel-inspired poly(dopamine)nanoparticle ink and Ag electroless plating. The process yields silverlines with thickness linearly dependant on plating time and electricalconductivity of 5× that of bulk silver. The conductive path could beadjusted between the twin lines and entire structures by controlling theELP time, which opens an opportunity for resolution control of printedconductive structures. This process demonstrates a simple,low-temperature and low-cost method of fabricating conductive paths forflexible electronics applications.

There has thus been shown and described novel communications devices andsystems and methods which fulfil various objects and advantages soughttherefor. Many changes, modifications, variations, combinations,subcombinations and other uses and applications of the subject inventionwill, however, become apparent to those skilled in the art afterconsidering this specification and the accompanying drawings whichdisclose the preferred embodiments thereof. All such changes,modifications, variations and other uses and applications which do notdepart from the spirit and scope of the invention are deemed to becovered by the invention, which is to be limited only by the claimswhich follow.

REFERENCES

Each of the following references is expressly incorporated herein byreference in their entirety.

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What is claimed is:
 1. A method for forming a conductive trace on asubstrate wettable with a volatile liquid, comprising: providing asuspension of catalytic nanoparticles in the volatile liquid, thecatalytic nanoparticles comprising a catalyst for electroless plating;selectively depositing the suspension of catalytic nanoparticles in thevolatile liquid on the substrate by printing a line in a pattern havingedges, wherein at least a portion of the substrate remains un-wet by thevolatile liquid; drying the volatile liquid, to form a pattern ofcatalytic nanoparticles on the substrate, by evaporation of the volatileliquid to redistribute the catalytic nanoparticles according to thecoffee ring effect to induce a higher concentration of catalyticnanoparticles in a first region on the substrate at the edges of theprinted line and a sparse distribution of catalytic nanoparticles at asecond region between the edges of the printed line, and wherein theun-wet portion of the substrate in a third region lacks catalyticnanoparticles; and selectively electroless plating the nanoparticles, toform a conductive metal pattern corresponding to a distribution of theformed pattern of catalytic nanoparticles in the first region, anon-conductive metal pattern corresponding to the distribution of theformed pattern of catalytic nanoparticles in the second region, and anabsent metal pattern the third region.
 2. The method according to claim1, wherein the substrate comprises at least one of a polyimide sheet andglass.
 3. The method according to claim 1, wherein the catalyticnanoparticles comprise polydopamine.
 4. The method according to claim 1,wherein the suspension of catalytic nanoparticles is selectivelydeposited by a method selected from the group consisting of inkjetprinting, lithography, and a pad printing process.
 5. The methodaccording to claim 1, wherein said drying comprises evaporating thevolatile liquid in the suspension at an exposed surface in air.
 6. Themethod according to claim 1, wherein the electroless plating comprisesplating the catalytic nanoparticles with at least one of silver andcopper.
 7. The method according to claim 1, wherein the electrolessplating forms a pair of separated conductive traces at the edges of aninkjet printed line of the catalytic nanoparticles.
 8. The methodaccording to claim 1, wherein the electroless plating compriseselectroless copper plating, resulting in at least one continuous reducedcopper trace on the substrate.
 9. A patterned substrate having aconductive trace on a substrate wettable with a volatile liquid, formedby a method comprising: selectively depositing a suspension of catalyticnanoparticles in the volatile liquid, the catalytic nanoparticlescomprising a catalyst for electroless plating, on the substrate byprinting a line in a pattern having edges, wherein at least a portion ofthe substrate remains un-wet by the volatile liquid; drying the volatileliquid, to form a pattern of catalytic nanoparticles on the substrate,by evaporation of the volatile liquid to redistribute the catalyticnanoparticles according to the coffee ring effect to induce a higherconcentration of catalytic nanoparticles in a first region on thesubstrate at the edges of the printed line and a sparse distribution ofcatalytic nanoparticles at a second region between the edges of theprinted line, and wherein the un-wet portion of the substrate in a thirdregion lacks catalytic nanoparticles; and selectively electrolessplating the nanoparticles, to form a conductive metal patterncorresponding to a distribution of the formed pattern of catalyticnanoparticles in the first region, a non-conductive metal patterncorresponding to the distribution of the formed pattern of catalyticnanoparticles in the second region, and an absent metal pattern thethird region.
 10. The patterned substrate according to claim 9, whereinthe substrate comprises at least one of a polyimide sheet and glass. 11.The patterned substrate according to claim 9, wherein the catalyticnanoparticles comprise polydopamine.
 12. The patterned substrateaccording to claim 9, wherein the suspension of catalytic nanoparticlesis selectively deposited by a method selected from the group consistingof inkjet printing, lithography, and a pad printing process.
 13. Thepatterned substrate according to claim 9, wherein the electrolessplating comprises plating the catalytic nanoparticles with at least oneof silver and copper.
 14. The patterned substrate according to claim 9,wherein the selectively depositing the suspension of catalyticnanoparticles comprises inkjet printing a line, to form a pair ofseparated conductive traces at the edges of the inkjet printed line. 15.The patterned substrate according to claim 9, wherein the electrolessplating comprises electroless copper plating, resulting in at least onecontinuous reduced copper trace on the substrate.
 16. A method forforming a conductive trace on a substrate, comprising: providing asuspension of catalytic nanoparticles adapted for electroless plating ina volatile liquid; depositing the suspension of catalytic nanoparticleson the substrate in a wetting pattern, wherein at least a portion of thesubstrate remains dry and absent catalytic nanoparticles; drying thevolatile liquid, to selectively form a pattern of catalyticnanoparticles on the substrate corresponding to the wetting pattern, andhaving an inhomogeneous distribution of catalytic nanoparticles due toredistribution of the catalytic nanoparticles during the drying,comprising a dense region of catalytic nanoparticles at edges of thewetting pattern, and a sparse distribution of catalytic nanoparticlesbetween the edges of the wetting pattern, and a region that remained dryoutside the wetting pattern without catalytic nanoparticles; andelectroless plating the catalytic nanoparticles, to form a conductivemetal pattern corresponding to the inhomogeneous distribution of thecatalytic nanoparticles, comprising a conductive metallized regioncorresponding to the dense region, and a non-conductive regioncorresponding to the sparse distribution, separating portions of theconductive metallized region.
 17. The method according to claim 16,wherein the catalytic nanoparticles comprise polydopamine.
 18. Themethod according to claim 16, wherein the suspension of catalyticnanoparticles is selectively deposited by a method selected from thegroup consisting of inkjet printing, lithography, and a pad printingprocess.
 19. The method according to claim 16, wherein the electrolessplating comprises plating the catalytic nanoparticles with at least oneof silver and copper.
 20. The method according to claim 19, wherein theelectroless plating forms a pair of separated conductive traces at theedges of an inkjet printed line of the catalytic nanoparticles.